1. Field of the Invention
The present invention relates to a technique for measuring a thickness of a thin film formed on a substrate.
2. Description of the Background Art
In recent years, as a circuit pattern of a semiconductor product has become finer based on a scaling law, a thickness of a film formed on a semiconductor substrate (which will hereinafter be referred to as a “substrate”) in semiconductor manufacturing processes has become smaller. It is expected that a film thickness of silicon oxide (SiO2) serving as a gate insulator is equal to or smaller than 1 nm for 65-nm technology node (i.e., a node length), for example, in the future.
On the other hand, it is well known that an optical measurement value of a film thickness is increased due to exposure of a substrate to an atmospheric air in a clean room or storage in a substrate container. This phenomenon is considered to be caused due to adsorption of organic contamination which is produced due to gas released from a plastic material or the like, onto a surface of the substrate. For example, it was confirmed that in a case where a substrate on which a film of silicon oxide with a thickness of 9.2 nm (p-type silicon (Si) substrate) is formed is stored in a substrate container for ten days, the thickness as measured after the storage is increased by approximately 0.2 nm. As such, as a film is becoming further thinner, increase in the thickness due to organic contamination shall more significantly affect process control of semiconductor manufacturing processes. It is additionally noted that though to employ a material which releases little gas as a substrate container for storing a substrate, or to provide a chemical filter, might be of some help to suppression of adsorption of organic contamination onto the substrate, it is difficult to completely eliminate released gas by the foregoing solutions.
Thus, suggested is a method of heating a substrate to remove adsorbed organic contamination prior to measuring a thickness of a film on the substrate, in order to accurately measure the thickness of the film. For example, the specification of U.S. Pat. No. 6,261,853 (which will be hereinafter referred to as “Reference I”) discloses a method in which organic contamination is removed from a substrate by heating the substrate in a heating chamber, the substrate is cooled in a cooling chamber which is thermally separated from the heating chamber, and a thickness of a film on the substrate is measured. Also, the specification of U.S. Pat. No. 6,519,045 (which will be hereinafter referred to as “Reference II”) discloses a method in which a difference between values of thickness which are measured (measurement value) before and after heating a substrate is previously calculated as a correction value, and the correction value is subtracted from a measurement value of a different substrate without heating the different substrate, to employ the resultant value as a true film thickness. However, as shown in A. Danel and four other persons, “Dry Cleaning of Organic Contamination on Silicon Wafers Using Rapid Optical Surface Treatment” (Switzerland), published by Scitec Publications, Solid State Phenomena, Vols. 76-77, 2001, pp. 59-62, it was confirmed that organic contamination cannot be completely removed by heating a substrate. Further, there is another known method for removing organic contamination, which utilizes ultraviolet rays or ozone. However, this method has the possibility of deteriorating a film formed on a substrate.
In view of the above-mentioned fact described by A. Danel et al., it can be considered that accurate measurement of a thickness of a thin film cannot be achieved only by measuring a thickness of thin film on a substrate after heating the substrate as in the method disclosed by Reference I, because of influences of organic contamination remaining on the substrate. Also, in the method disclosed by Reference II, influences of organic contamination remaining on a substrate after the substrate is heated are not taken into account the correction value. Further, it was confirmed that an amount of adsorbed organic contamination varies depending on each portion of one substrate or on each of substrates stored in one substrate container accommodating the substrates. As such, it is impossible to accurately obtain a thickness of a thin film on a substrate by the method disclosed in Reference II.